The recording density of a hard disk drive (HDD), which is a type of magnetic recording/reproducing apparatus, is increasing at an annual rate of 50% or higher, and this increasing trend is expected to continue. In response to this, magnetic heads and magnetic recording media that are suitable for high recording density are being developed.
A magnetic recording medium equipped in a magnetic recording/reproducing apparatus currently available on the market is a so-called perpendicular magnetic recording medium in which the magnetization easy axis in the magnetic film is primarily oriented perpendicular. In a perpendicular magnetic recording medium, even when its recording density is highly increased, diamagnetic field influence in a boundary region between recording bits is small and clear bit boundaries are formed, and therefore, an increase in noise can be suppressed. In addition, it has a small decrease in recording bit volumes when highly increasing recording density, and consequently is resistant to thermal fluctuation effects. For this reason, perpendicular magnetic recording media are receiving broad attention in recent years, and there has been proposed a structure of a medium that is suitable for perpendicular magnetic recording.
Moreover, in order to respond to the demand for further increasing the recording density of a magnetic recording medium, use of a single magnetic pole head, which has a superior ability to write into a perpendicular magnetic layer, is being considered. In order to adapt to such a single magnetic pole head, there has been proposed a magnetic recording medium that improves incoming/outgoing efficiency of magnetic flux between a single magnetic pole head and a magnetic recording medium by providing a layer called a backing layer composed of a soft magnetic material between a perpendicular magnetic layer that serves as a recording layer and a non-magnetic substrate.
However, in a magnetic recording/reproducing apparatus that uses the above-mentioned magnetic recording medium simply having a backing layer provided therein, recording/reproducing characteristics when recording and reproducing, resistance to thermal fluctuations, and recording resolution are not satisfactory, and consequently there has been a demand for a magnetic recording medium that excels in these characteristics.
In order to further increase recording density in the future, it is essential to: achieve high S/N ratio, which is a ratio of signal to noise (S/N ratio) in reproduction and is particularly important as a recording/reproducing characteristic; and improve thermal fluctuation resistance. However, these two items are in a conflicting relationship, in which if one of them is improved, then the other one will be insufficient, and therefore, achieving both of them at a high level is a significant problem.
In order to solve such a problem, there has been proposed a magnetic recording medium that is characterized in that three magnetic layers are AFC (anti-ferromagnetic coupling) bonded using a non-magnetic layer and the like, and thereby advantage of the reduction in composite Mrt and PW50 is taken without causing a reduction in S/N ratio (for example, refer to Patent Document 1).
Specifically, this Patent Document 1 discloses a magnetic recording medium that is characterized in that it has: a substrate; a first lower ferromagnetic layer that is provided on the substrate and has a residual magnetization Mr, a thickness t, and a product Mrt of residual magnetization times thickness; a ferromagnetic bonding layer that is provided on the first lower ferromagnetic layer; a second lower ferromagnetic layer that is provided on the ferromagnetic bonding layer and has an Mrt value; an anti-ferromagnetic bonding layer that is provided on the second lower ferromagnetic layer; and an upper ferromagnetic layer that is provided on the anti-ferromagnetic bonding layer and has an Mrt value greater than that of the total Mrt value of the first and second lower ferromagnetic layers.
Meanwhile, in order to improve the recording/reproducing characteristic and thermal fluctuation characteristic of a perpendicular magnetic recording medium, there has been proposed to use an orientation control layer and form multiple magnetic layers so as to prepare crystal grains of each magnetic layer into consecutive column-shaped crystals, thereby enhancing the perpendicular orientation of the magnetic layers (for example, refer to Patent Document 2).
Moreover, for example, use of Ru as the orientation control layer has been disclosed. The apex section of a column-shaped crystal of Ru has a dome-shaped convex section, and therefore Ru has an effect such that crystal grains of a magnetic layer and the like are grown on this convex section, isolated structure of the grown crystal grains is promoted, the crystal grains are isolated, and magnetic grains are grown into a column shape (for example, refer to Patent Document 3).